The MRI apparatus is an apparatus that measures a nuclear magnetic resonance (NMR) signal, which is generated by applying a high-frequency magnetic field pulse to an object to be tested in a state where the object to be tested is placed in a uniform static magnetic field, and reconstructs an image of the object to be tested by calculation of the NMR signal. An image with a high SN can be obtained by using a high magnetic field generator as a static magnetic field in which the object to be tested is placed.
In recent years, a high magnetic field MRI apparatus capable of realizing a high magnetic field of 3 T or more has become widespread with the development of superconducting magnets. In the high magnetic field MRI, a high SN is obtained, but there is a problem in that brightness unevenness occurs in an image in abdominal imaging or the like. As one of the causes of this brightness unevenness, there is spatial non-uniformity of the magnetic field distribution (irradiation magnetic field distribution, B1 distribution) of high-frequency magnetic field pulses (hereinafter, referred to as RF pulses) to excite the nuclear spins in the tissue of the object. Generally, the resonance frequency of the high-frequency magnetic field for excitation is proportional to the static magnetic field strength. Accordingly, in the high magnetic field MRI, it is necessary to radiate a magnetic field of a frequency higher than that in the high-frequency magnetic field in the related art. In this case, the wavelength of the high-frequency magnetic field in the body becomes the same scale as the size of the body (particularly, abdomen). For this reason, the phase of the high-frequency magnetic field changes depending on the position in the body, and this change appears as image unevenness.
There is an RF shimming as a technique for solving the spatial non-uniformity of the irradiation magnetic field distribution (B1 distribution). In the RF shimming, the non-uniformity of the B1 distribution is reduced by controlling independently the intensity and phase of the RF pulse given to each channel using an RF coil for transmission with a plurality of channels. In order to determine the intensity and phase of the RF pulse given to each channel, the B1 distribution of each channel is required for each object and each imaging part, and various methods for measuring the B1 distribution have been proposed.
A typical method for measuring (B1 measurement) the B1 distribution is a method called a Double Angle Method (DAM), and the B1 distribution is measured by calculation of an image captured using an RF pulse with an arbitrary flip angle and an image captured using an RF pulse with a flip angle which is twice the arbitrary flip angle. In addition, a method for calculating the B1 distribution by acquiring a plurality of images using a plurality of RF pulses with different flip angles and performing the fitting of the signal intensities (pixel values) of the plurality of images according to the signal intensity expression determined by the pulse sequence has also been proposed (NPL 1). In addition, for a plurality of images acquired similarly, a method for calculating the B1 distribution from the period of signal intensity change has also been proposed (PTL 1).
In addition, a method for repeating the pulse sequence, which is for acquiring an image by applying a high-frequency magnetic field pre-pulse (hereinafter, simply referred to as a pre-pulse), while changing the intensity of the pre-pulse and calculating the B1 distribution from the acquired image (NPL 2) or a method for calculating the B1 distribution by taking the ratio between an image acquired immediately after pre-pulse application and an image acquired without applying the pre-pulse has also been proposed (NPL 3).